1. Field of The Invention
The present invention relates to an apparatus for producing electrolytic water having an excellent cleaning effect, i.e., anode water (acidic water) and/or cathode water (alkaline water). More particularly, this invention relates to an apparatus for producing anode water and/or cathode water which both have an excellent cleaning effect and may be used for cleaning electronic devices, e.g., semiconductors and liquid crystals.
2. Description of the Related Art
In producing and cleaning highly integrated electronic parts, such as liquid crystals and semiconductor wafers, cleaning media specially prepared for these purposes have conventionally been used, such as, e.g., sulfuric acid, hydrofluoric acid, hydrogen peroxide, and hydrochloric acid. These cleaning media will continue to be suitably used for certain applications. However, since these cleaning media are obtained by specially purifying corresponding products produced through chemical processes, the purification operations are complicated because they involve removing metallic ingredients that have come into contact with the chemical products, for example, from the catalysts used for producing the products. As a result, the purified products are expensive. In addition, even if the purification operations are conducted carefully, the thus-purified products are not always sufficiently pure in view of the reduced amount of allowable impurities resulting from the progress of electronic devices. Now substitute techniques are hence desired.
One of these substitute techniques is to use ozonized water. In particular, highly ozonized water produced by electrolysis is known to be exceedingly effective, e.g., in cleaning electronic devices. However, since the use of ozonized water alone is insufficient in some cases, there is a growing desire for a treatment liquid that has one or more functions not possessed by ozonized water, e.g., an oxidizing function and a reducing function, and which contains no metallic ingredients.
One such treatment liquid is anode water or ultra-acidic water. Anode water generally has a pH of 2.7 or lower and an oxidation-reduction potential (ORP) of 1.1 V or higher and hence has an oxidizing ability. Consequently, anode water has the effect of, for example, decomposing organic substances or dissolving metallic deposits therein to remove these impurities, and is used for the cleaning of electronic devices, etc., although the amount used remains small.
Simultaneously with the production of the anode water in an electrolytic cell, cathode water having a pH of 10 or higher and an ORP of 0 V or lower is yielded as a by-product in the cathode chamber of the electrolytic cell. It has been reported that when electrolytic water having oxidizing properties or having reducing properties (the former being called acidic water or anode water and the latter being called cathode water or alkaline water) both produced through water electrolysis, which is a relatively simple operation, is used in place of a reagent such as a high-purity acid or alkali or hydrogen peroxide, then the water not only produces the same cleaning effect as the reagent but considerably reduces costs.
An ordinary electrolytic water, e.g., anode water, is easily obtained by conducting electrolysis while feeding dilute hydrochloric acid to an electrolytic cell as anolyte feed material. In the case of electrolysis, which generates ozone and other substances, water having a high ORP is obtained by merely feeding pure water to an electrolytic cell. It has also been reported that a target electrolytic water can be obtained at a lower cost by a technique in which chloride ions as a feed material are fed to a cathode chamber and are then sent to an anode chamber through a membrane.
The production efficiency of anode water and cathode water varies depending on the kind of feed water, the kind of electrode catalyst, and the operating conditions including current density. Effective anode water and cathode water preferably contain, in a high concentration, radicals that enhance the cleaning effect of the water.
In producing an electrolytic water to be used for semiconductor cleaning, it is necessary to minimize the inclusion of impurities generated in the electrolytic cell and the cell should not generate heavy-metal impurities attributable to an electrode. Among the preferred electrode catalysts capable of inhibiting the generation of such impurities is an iridium oxide catalyst. However, since this catalyst has an insufficient resistance to chlorine and ozone gases, which serve to stably generate radicals, the catalyst contributes little to an increase in cleaning effect. Although an electrolytic water having an excellent cleaning effect is obtained if radicals can be stably generated, the generation of radicals is influenced not only by the catalyst but by other electrolysis conditions such as the kind of feed water and current density.
In general water electrolysis, oxygen generates from the water on the anode side where the water is deprived of electrons (formula (1)). The addition of hydrochloric acid is intended to accelerate the generation of free chlorine and hypochlorous acid (formulae (2) and (3)). Ozone can generate depending on the catalyst selected (formula (4)). EQU 2H.sub.2 O.fwdarw.O.sub.2 +4H.sup.+ +4e (1) EQU 2Cl.sup.-.fwdarw.Cl.sub.2 +2e (2) EQU Cl.sub.2 +H.sub.2 O.fwdarw.HClO+H.sup.+ +Cl.sup.- (3) EQU 3H.sub.2 O.fwdarw.O.sub.3.sup.+ +6H.sup.+ +6e (4)
The hydrogen ions yielded by the anodic reaction shown by formula (1) are partly reduced to hydrogen on the cathode, and the other hydrogen ions remain in the anode to make the water acidic. A high ORP is derived from the ORP's of free chlorine and hypochlorous acid. Although the generation of oxygen proceeds in preference to chlorine generation from the standpoint of equilibrium, the latter reaction also proceeds on many electrodes. It is easy to conduct chlorine discharge in preference to oxygen discharge by suitably selecting a catalyst. The current efficiency in the oxidation reaction of chloride ions depends on concentration and pH.
Ozone is known to be generally unstable and react with water to yield active OH radicals and O radicals. It has however been reported that in pure water, ozone is fairly stable and has a half-life period as long as several hours. This indicates that radical generation by ozone decomposition requires a stimulus, such as a certain amount of impurities or ultraviolet exposure, and a catalyst. Consequently, an effective technique for generating radicals is to dissolve a slight amount of a catalyst or to fix, within a piping of the cell, a substance accelerating ozone decomposition so that this substance is in contact with an electrolytic water. However, these techniques are unnecessary where the ozone or active chlorine contained in the electrolytic water decomposes upon contact with fouling substances adhered to the parts being cleaned and thus OH and O radicals are generated.
In elementary processes of an electrode reaction, the adsorbed ions and water molecules generate an intermediate discharge species. Namely, chemical species corresponding to radicals are generated on the electrode surface. It may be thought that these chemical species react with solution molecules and thus generate radical species. Where chloride ions are present, the following reaction processes for radical generation are presumed. In the following formulae, each species included in parentheses denotes a chemical species present on the electrode surface, while each species marked with the symbol ".circle-solid." denotes a radical. EQU Cl.sup.-.fwdarw.(Cl)+e (5) EQU (Cl)+H.sub.2 O.fwdarw.ClOH.sup.- +H.sup.+ (6) EQU ClOH.sup.-.fwdarw.Cl.sup.- +.circle-solid.OH (7)
Furthermore, the following reaction processes for radical generation are expected, which begin with water oxidation.
H.sub.2 O.fwdarw.(OH)+H.sup.+ +e (8) EQU (OH)+Cl.sup.-.fwdarw.CLOH.sup.- (9) EQU ClOH.sup.-.fwdarw.Cl.sup.- +.circle-solid.OH (7)
Cathode water, which has a reducing ability, is originally derived from hydrogen gas. In an electrolytic cell having a cation-exchange membrane, hydrogen is generated on the cathode from either hydrogen ions that have moved from the anode side or the catholyte water.
On the other hand, hydrogen peroxide, which can be electrolytically generated directly or indirectly from oxygen, serves as a radical-generating material and is effective in cleaning, like ozone.
The electrolytic water thus produced by electrolysis, particularly those for use in the cleaning of semiconductors and the like, preferably do not contain ions and the like resulting from the counter-electrode chamber during electrolysis, nor do they contain the impurities described above, and thereby the quality of the water is improved.